The present invention relates to spectroscopy methods and apparatus, including both monochromator and spectrometer methods and apparatus, and particularly relates to methods of improving the signal-to-noise ratios therein.
Spectroscopy systems include monochromators and/or spectrometers. Monochromators are optical instruments designed to separate light consisting of a multiplicity of wavelengths into its separate components and to deliver light of a single wavelength or relatively narrow band of wavelengths to an optical system or an optical detector. Spectrometers are instruments used to disperse light, including light that may have interacted with an optical system, for the purpose of determining its intensity as a function of photon wavelength or energy. When used for characterization purposes, for example material characterization by reflection or transmission, monochromators and/or spectrometers are components of more general optical systems that typically include a source of illumination such as an incandescent or arc lamp as well as some means of directing the illumination from the source either onto the monochromator for subsequent interaction with the remainder of the system or onto the spectrometer after the light has interacted with the system.
Monochromators and spectrometers are used as components in a wide variety of industrial and scientific applications. For example, either a monochromator or a spectrometer is generally part of a spectroscopic ellipsometer, an optical system that is widely used to characterize bulk materials, thin films, surfaces, and interfaces for such properties as composition, microstructure, and thickness. See, e. g., Aspnes et al., U.S. Pat. No. 5,877,859 and Aspnes et al., U.S. Pat. No. 5,798,837.
The performance or signal-to-noise ratio of an optical system containing a broadband or white-light source is theoretically limited only by shot noise, the intrinsic statistical fluctuation of the number of photons recorded by the detector at a given wavelength or range of wavelengths per unit time. However, numerous other effects, such as mechanical vibration of the system components, fluctuations in refractive index of air in the vicinity of the source, or arc wander in the case of an arc lamp, can all act to increase noise and therefore to degrade the signal-to-noise ratio. The best performance is generally realized by incandescent lamps having relatively large area filaments that emit light when heated to a high temperature. Unfortunately, owing to intrinsic limitations of the filament material, the wavelengths available from incandescent sources are not adequate for many applications and arc lamps, which can emit at substantially shorter wavelengths, must be used, usually at a cost of signal-to-noise ratio.
There is a need to improve signal-to-noise ratios of spectroscopy systems that use arc lamps. By improving signal-to-noise ratios, weaker signals can be detected, strong signals can be measured more quickly, and the performance of optical systems can be improved in general. Unfortunately, current spectroscopy systems that employ arc lamps are at a point where further improvements in signal-to-noise ratios are difficult to realize.
The present inventors unexpectedly found that the signal-to-noise ratio of an optical system employing an arc-lamp source and a spectrometer could be improved substantially by orienting the major axis of the arc lamp, i.e., the axis parallel to its electrodes, such that the image of its major axis at the entrance slit of the spectrometer is orthogonal to the major axis of the slit, i.e., to the long dimension of the slit. Insofar as the inventors are aware, in prior art the major axis of the arc lamp (and its image at the slit) is oriented generally parallel to the major axis of the slit. This is probably done because slits tend to be oriented vertically and arc lamps are generally oriented vertically.
A first aspect of the present invention is a spectroscopy system exhibiting less noise and therefore an enhanced signal-to-noise ratio. This system comprises:
(i) an arc lamp light source of emitted light, which emitted light is projected as an image of the light source and then interacts with the rest of the optical system;
(ii) a slit aperture through which the emitted light that has interacted with the rest of the optical system is projected; and
(iii) a detector operably associated with the slit aperture for detecting the light after passing through the spectrometer.
The light projected onto the slit is an image of the arc lamp light source. By orienting the major axis of the image essentially orthogonal to the major axis of the slit, accomplished here by physically rotating the arc lamp, it was observed that the signal-to-noise ratio of the output signal from the detector was improved significantly relative to its value when the major axis of the image was essentially parallel to the slit. Thus the signal-to-noise ratio of the system is improved relative to its value when the major axis of the slit aperture is oriented parallel to the major axis of the image of the arc lamp.
A second aspect of the present invention is a method of enhancing the signal-to-noise ratio of a spectrometer, the method comprises:
(i) providing a spectrometer having an arc lamp light source of emitted light, which emitted light is projected as an image of the light source; a slit aperture through which the emitted light is projected; and a detector operably associated with the slit aperture for detecting the emitted light; wherein both the slit aperture, the arc lamp, and the image of the arc lamp each have a major axis; and
(ii) orienting the major axis of the slit aperture essentially orthogonally to the major axis of the image of the arc lamp, so that the signal-to-noise ratio of the spectrometer is improved as compared to the signal-to-noise ratio of the spectrometer when the major axis of the slit aperture is oriented parallel to the major axis of the image of the arc lamp.
Without wishing to be bound to any particular theory of the invention, it is believed that the reason for the improvement is that in any arc lamp the arc within the lamp exhibits some wander or movement, which occurs mainly perpendicular to the major axis of the lamp. The arc wander occurs because the so-called xe2x80x9chot spotxe2x80x9d from which the major fraction of the intensity originates is located at the positive electrode, the anode, which is the larger of the two electrodes of the lamp. As the lamp operates, motion of the hot spot is more likely to occur around the anode, i.e., orthogonally to the axis of the electrodes (the axis of the lamp), since in operation the hot spot remains at a fixed distance from the electrode. If the image of the arc at the entrance slit of the monochromator or spectrometer is larger than the width of the slit, owing to nonuniformities in intensity over the arc any motion is translated into a change of intensity and thus increased noise. However, if this motion is along the slit instead of perpendicular to it, the intensity change, and hence the generated noise, is clearly much less. The same comments apply to the situation where the xe2x80x9cslitxe2x80x9d may be defined by pixels in a diode-array detector.
The foregoing and other objects and aspects of the present invention are explained in greater detail in the drawings herein and the specification set forth below, in which: